LASER PHOTOACOUSTIC SPECTROSCOPY OF FORBIDDEN TRANSITIONS: ACETYLENE AND ALKYNE HIGH ENERGY VIBRATIONAL STATES AND THEIR INTERACTIONS (INTRAMOLECULAR, REDISTRIBUTION)
HALL, RANDY RAY
Doctor of Philosophy
We developed an ultrasensitive, computer-controlled, visible/near-IR, cw laser photoacoustic spectrometer capable of detecting forbidden transitions in order to understand the nature and states of highly vibrationally excited molecules. We have obtained nearly Doppler-limited survey spectra of highly vibrationally excited alkynes over a considerable energy range. Acetylene and substituted acetylenes (HCCR) comprise a novel group of molecules because the highly localized CH chromophore (CH oscillator) is linearly coupled via a strong CC triple bond to the rest of the molecule. Thus, these HCCR alkynes are essentially one dimensional molecules particularily amenable to studies involving intramolecular energy redistribution. We first studied acetylene (H('12)C('12)Ch) and four isotopic modifications (H('12)C('13)CH, H('13)C('13)CH, H('12)C('12)CD, D('12)C('12)CD) to understand the global pattern (spectral positions, assignments, intensities, and perturbations) of vibrational transitions in simple, model polyatomic molecules at high levels of vibrational excitation and to develop comparison data for spectral analyses of substituted acetylenes which exhibit intramolecular vibrational mixing and energy redistribution. The second phase of our studies was to obtain detailed spectroscopic information on the overtones of the following substituted acetylenes (HCCR): cyanoacetylene (HCCCN), methylacetylene (HCCCH(,3)), 3,3,3-trideuteromethylacetylene (HCCCD(,3)), 3,3,3-trifluoromethylacetylene (HCCCF(,3)), and tertiary butyl acetylene (HCCC(CH(,3))(,3)). The substituent group "R" was varied from a simple H atom to a tertiary butyl group to study not only the effect of increasing density of states but to understand and distinguish the necessary and sufficient causes of irreversible intramolecular energy redistribution in highly vibrationally excited states of isolated polyatomic molecules as a function of increasing molecular complexity.